It has been recognized that there is a need in the operation of heavy duty trucks for increased braking performance and safety, and longer life of the wheel brake linings. In the past coil-type electromagnetic retarders and hydro-dynamic retarders have been used on drive shafts to supplement direct wheel braking. More recently it has been proposed to use strong permanent magnets instead of electromagnets in drive shaft retarders. In these braking devices a drum containing ferrous pole pieces on its inner side is mounted as a rotor on the drive shaft from the engine. A yoke containing a ring of permanent magnets is slide mounted on the track chassis to move coaxially relative to the drive shaft so as to position the magnets opposite the ferrous pole pieces on the drum with a fixed narrow air gap therebetween. The braking torque for a given width of air gap and rotor speed is dependent upon the amount of overlap of the magnets with the pole pieces. When the retarder is in an "off" condition the permanent magnets are moved out of attraction range for the ferrous poles. The drum has external cooling fins to assist in dissipating the substantial heat generated as the brake energy is absorbed generating high ampere eddy currents which are converted to thermal energy heating the drum sometimes to over 600.degree. C.
The described permanent magnet type retarder has been considered to having size and weight advantages over the prior art and to require less chassis modification. However, it does not utilized permanent magnets in the most effective manner for the intended purpose. The present invention aims to provide an improved permanent magnet braking system not involving the attraction of magnets for ferrous poles, but, instead, utilizing the drag (magnetic friction) resulting from relative rotary movement between permanent magnets and an electroconductive pole plate (such as a copper plate) separated by an air gap. It has been found that such an arrangement can take advantage of the superior thermal conductivity of the pole plate for heat dissipation, decrease movement of the magnets for on-off control, and maximize effective use of the magnets.